Prices in Singapore for cameras whether new or secondhand are no longer cheap. Don't go out with the expectation of finding the bargain of a lifetime.
My advice would be to know beforehand what camera you are looking for and buy only if you find one that suits you. Condition and cosmetics are everything!
This said, drop in at Camera Hospital, 91 Bencoolen Street (Sunshine Plaza) in the city-center. See the truly amazing double window full of old cameras - owner Steven Lee appears to have cornered the local market on Rollei TLRs among many other 'oldies' - and while I've never bought a camera from him, several friends have and they all recommend him very highly as a seller of top quality gear.
I've always found him to be a genial, pleasant and informative source of good advice on what ails your camera.
Steven is highly recommended by many Singaporeans for any camera repairs you may need. Fast and reliable service, and he will often do the work for you in 1-2 days or even on the spot (depending on what ails your beast) if you are short on time.
Check out Steven's web site, www.camerahospital.com